Prefix Sum Array in DSA Python - Build from Scratch

Prefix Sum Array

📖 Scenario: You are working with daily sales data for a small shop. You want to quickly find the total sales from the start of the month up to any given day.

🎯 Goal: Build a prefix sum array that stores cumulative sales totals for each day. This will help you answer total sales queries efficiently.

📋 What You'll Learn

Create a list of daily sales numbers

Create a prefix sum list to store cumulative totals

Use a loop to fill the prefix sum list

Print the prefix sum list

💡 Why This Matters

🌍 Real World

Prefix sum arrays are used in finance, data analysis, and gaming to quickly calculate totals over ranges without repeated addition.

💼 Career

Understanding prefix sums helps in optimizing algorithms and is a common interview topic for software engineering roles.

1

Create the daily sales list

Create a list called daily_sales with these exact values: [10, 20, 15, 30, 25]

DSA Python

# Create the daily_sales list with the given values
# Your code here

Hint

Use square brackets and commas to create the list.

2

Create the prefix sum list

Create a list called prefix_sum with the same length as daily_sales, filled with zeros.

DSA Python

daily_sales = [10, 20, 15, 30, 25]
# Create prefix_sum list with zeros, same length as daily_sales
# Your code here

Hint

Use len() to get the length and multiply a list with zero.

3

Fill the prefix sum list with cumulative totals

Use a for loop with variable i from 0 to len(daily_sales) to fill prefix_sum so that each element is the sum of all previous sales including the current day. For i == 0, set prefix_sum[0] to daily_sales[0]. For others, set prefix_sum[i] to prefix_sum[i-1] + daily_sales[i].

DSA Python

daily_sales = [10, 20, 15, 30, 25]
prefix_sum = [0] * len(daily_sales)
# Fill prefix_sum with cumulative sums using a for loop
# Your code here

Hint

Use range(len(daily_sales)) and an if condition inside the loop.

4

Print the prefix sum list

Print the prefix_sum list to display the cumulative sales totals.

DSA Python

daily_sales = [10, 20, 15, 30, 25]
prefix_sum = [0] * len(daily_sales)
for i in range(len(daily_sales)):
    if i == 0:
        prefix_sum[i] = daily_sales[i]
    else:
        prefix_sum[i] = prefix_sum[i-1] + daily_sales[i]
# Print the prefix_sum list
# Your code here

Hint

Use print(prefix_sum) to show the list.